<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form autocomplete="off">
    姓名:<input type="text" id="username">
    <span id="uSpan"></span>

    <br>
    密码:<input type="password" id="password">
    <br>
    <input type="submit" value="注册">

</form>
</body>
<script src="js/jquery-3.3.1.min.js"></script>
<script>
    $("#username").blur(function(){
        let usernameVal=this.value;
        $.get("userServlet",{username:usernameVal,age:18},function(data){
            let m=data.msg;
            alert(m);
        },"json");
    })
</script>
</html>